In conventional-scale liquid chromatography, the mobile phase liquid is usually conveyed between components of the chromatography system in tubing constructed from stainless steel, poly-ether-ether-ketone (PEEK™), or fluoridated hydrocarbon polymers such as those marketed using trademarks like Teflon® (Dupont). Conventional chromatography is typically practiced with analytical columns having a typical internal diameter in the range of 3.9 to 4.6 millimeters. A typical external diameter for interconnection tubing is nominal 1/16th inch (approximately 0.062″ or 1.57 mm). The internal diameter of the interconnection tubing will generally vary with the type of application, but diameters ranging from 0.005″ to 0.040″ (0.127 to 1.02 mm) are common.
In recent years, interest has grown in the practice of liquid chromatography at capillary size scales. Capillary tubing is typically formed from fused silica (a type of glass) and will have an outside diameter in the range of 360 microns and an inside diameter in the range of 25 to 100 microns. The internal diameter of a capillary scale analytical column may range from 800 microns to 50 microns or less. For a column of 75 micron internal diameter, the volume of an eluting zone or band will typically be less than 100 nanoliter. In order to effect connections of a system incorporating a 75 micron diameter column, the connecting tubing will typically be chosen to have an internal diameter of 25 microns or less. As capillary size has decreased, fluid pressure in fluid handling devices has generally increased.
These dynamics have put tremendous stress on capillary fittings. One difficulty at high pressures has been simply retaining a capillary in a fitting.
Typically, a capillary tube is held in a fitting by a ferrule and compression screw that presses on the ferrule to provide the fluidic seal between the capillary and whatever it is joined with. No additional mechanisms for holding the capillary tube other than with the grip provided by the sealing ferrule in such a fitting have been required for high pressures (5,000 to 30,000 psi). However, as the pressure increases in fluid handling systems, a more secure gripping of the capillary in a fitting has become necessary. The ferrule/compression screw design provides insufficient retention of a capillary at ultra high pressure (30,000 to 100,000 psi). The insufficient clamping force of the ferrule/compression screw design results in the capillary being ejected from the fitting as fluid pressure is increased.